1. Field of the Invention
This invention relates generally to information signal reproducing apparatus, and more particularly, to apparatus for reproducing information signals recorded on a record medium in the form of digital data encoded together with data for error correction, wherein the digital data read out from the record medium are subjected to the error correction and decoded in order to be transformed into reproduced information signals.
2. Description of the Prior Art
There has been proposed a video disc having a spiral record track in for each turn of the disc a frame period segment of a video signal is recorded in such a manner as to be optically readable and from which a reproduced video signal of high quality can be easily obtained. The reproduction of the video signal from the video disc is performed by a video disc player in which the video disc is rotated at a predetermined rotating speed and the spiral record track on the video disc is scanned by an optical stylus formed with a light beam, such as a laser beam, provided in the video disc player, so that the video signal recorded in the spiral record track is read out.
In such reproduction of the video signal from the video disc, a reproduced video signal for a still picture can be obtained by reading one frame period segment of the video signal repeatedly from one of the turns of the spiral record track on the video disc. In this connection, there has been also proposed a system for obtaining video signals for reproducing still pictures together with audio signals for reproducing sound corresponding to the respective still pictures. In such a system, a unit audio signal corresponding to a frame period segment of the video signal for a still picture, that is, a still picture video signal of one frame period, is recorded in a plurality of turns of the spiral record track on the video disc preceding the turn of the spiral record track in which the still picture video signal is recorded. In a reproducing operation, the unit audio signal is first read and stored in a memory device, and then the corresponding still picture video signal is reproduced repeatedly to obtain a still picture and simultaneously the unit audio signal stored in the memory device is read from the memory to produce the sound for the reproduced still picture. In such a case, to reduce the number of the turns of the spiral record track used for recording the unit audio signal, each unit audio signal is compressed in time base when it is recorded and is then expanded during time base in reproduction. To obtain a reproduced sound satisfactory to practical use from the recorded unit audio signal that is compressed in time base on a large scale, each unit audio signal may be recorded in the form of audio digital data encoded in accordance with the PCM(pulse code modulation) coding.
On the video disc having the spiral record track in which the still picture video signals and the audio digital data corresponding to the respective still picture video signals are recorded, as shown in FIG. 1, for example, a recorded area AR is provided where audio digital data SFA are recorded in a plurality of turns of the spiral record track (each turn is indicated with T) and a recorded area VR is provided where a still picture video signal V of one frame period is recorded in each of three successive turns of the spiral record track and which follows the recorded area AR. A plurality of such combinations of recorded areas AR and VR may be provided successively with different audio digital data and different still picture video signals.
In reproducing signals from such a video disc, initially the plural turns of the spiral record track in the recorded area AR are successively scanned by the optical stylus so that the audio digital data SFA recorded therein are read out and stored in the memory device, and then one of the successive three turns at the center in the recorded area VR is repeatedly scanned by the optical stylus so that the still picture video signal V recorded therein is reproduced repeatedly. Thus, a still picture is obtained and simultaneously the audio digital data SFA stored in the memory device is read from the memory device and decoded through expansion in time base so that the sound for the reproduced still picture is obtained.
Such audio digital data SFA as mentioned above can also be contained in a quasi-video signal that can be recorded on the video disc and which has horizontal and vertical synchronous signals in similar to those in the still picture video signal. Assuming that the nth (n is a positive integer) one of horizontal periods successive to a vertical synchronous signal SV in each frame period of this quasi-video signal is represented by the nH, the audio digital data SFA contained in the quasi-video signal are positioned, as shown in FIG. 2, in successive ones of 240 horizontal periods from 23H to the 262H in the first field period and successive 240 horizontal periods from 285H to 524H in the second field period in each frame period.
In the quasi-video signal shown in FIG. 2, the horizontal synchronous signal is not shown and address data Fn are contained in 17H and 18H in the first field period in each frame period, which indicates the track turn number for a turn of the spiral track in which each frame period segment of the quasi-video signal is to be recorded For example, less than thirty-three frame periods of the quasi-video signal continues to be recorded in the recorded area AR, and therefore a unit quasi-video signal is formed with less than thirty-three frame period segments each containing the audio digital data SFA. As for each unit quasi-video signal containing less than thirty-three frame periods, in 21H and 22H in the first field period of the first frame period, header mark data Hm are positioned as indicated with a dot-dash line in FIG. 2, and in 525H in the second field period of the final frame period, trailer mark data Tm are positioned as indicated also with a dot-dash line in FIG. 2.
The audio digital data SFA contained in each unit quasi-video signal which is formed with less than thirty-three frame periods as described above are provided by encoding audio information data through an interleave in accordance with a cross-interleave technique to which the audio information data is subjected together with an error correcting code added thereto so that error correction is effectively carried out on the occasion of decoding. Such encoding of the audio information data to produce the audio digital data SFA is so performed that a predetermined coding format is established. In this predetermined coding format, for example, the audio digital data SFA which are contained in each of the frame periods of the unit quasi-video signal are encoded into successive 480 data segments DS each formed with 16 bytes (128 bits) and set to be contained in one horizontal period, as shown in FIG. 3 (In FIG. 3, the ends of the respective data segments DS are aligned vertically for the convenience of illustration). These successive 480 data segments DS as a whole constitute a data frame DF, and each data segment DS contains leader mark data Lm of one byte at the beginning end thereof and a data portion of fifteen bytes following the leader mark data Lm.
Then, as shown, in FIG. 4, less than thirty-three successive data frames DF constitute a data group DU in which each data frame DF corresponds to each of the frame periods of the unit quasi-video signal. At the begining end of each data frame DF, track turn number data Ut of thirty-two bytes representing the number of the turns of the spiral track used for recording the data group DU to which the data frame DF belongs are provided, and at the first half of the first data frame DF, control program data Pd are provided so that audio information data can be processed in accordance with the control program data Pd on the occasion of reproduction.
In such encoding of the audio digital data SFA as to constitute the coding format as mentioned above, the interleave to which the audio digital data SFA are subjected is carried out in such a manner that, for example, a plurality of data blocks each formed with thirty-two bytes are partitioned and a change of bit arrangement among a predetermined number of selected ones of the data blocks is achieved within each data group DU containing less than thirty-three data frames. That is, each data group DU in its entirety constitutes a unit of data wherein the interleave to which the audio digital data SFA are subjected is completed.
Accordingly, when the quasi-video signal containing the audio digital data SFA is read from the spiral record track on the video disc to reproduce the audio digital data SFA and the reproduced audio digital data SFA are decoded to reproduce the audio signal, the data group DU containing the less than thirty-three successive data frames DF is stored entirely in the memory device. Then, the predetermined number of the data blocks each formed with thirty-two bytes are selectively read from the memory device to be supplied to a decoder and the error correction is performed through a de-interleave carried out for solving the interleave for the data blocks supplied to the decoder, so that decoded audio information data corresponding to one data block are obtained from the decoder. These decoded audio information data are subjected to digital to analog conversion in a predetermined processing time (for example, 6 milliseconds) and consequently a reproduced analog audio signal segment corresponding to one data block is obtained.
After that, whenever one of the data blocks is newly read from the memory device in which the data group DU is stored entirely to be supplied to the decoder, the error correction is performed through the de-interleave carried out for solving the interleave for the data blocks supplied to the decoder so as to produce newly decoded audio information data corresponding to one data block. These decoded audio information data are also subjected to the digital to analog conversion in the predetermined processing time and consequently a new analog audio signal segment corresponding to one data block is reproduced. Is such a manner as mentioned above, the reproduced audio signal segments each corresponding to one data block of the audio information data are successively obtained and, as a result, a continuous analog audio signal is reproduced.
In reproducing the audio signal from the audio digital data SFA read from the video disc in such a manner as aforementioned, if the audio digital data SFA, which are selectively read one data block at a time from the memory device in which the data group DU of the audio digital data SFA is stored entirely, contain a certain bit error which prevents the error correction for the audio digital data SFA in the decoder from being carried out in a specific manner, that is, a situation wherein the error correction in the decoder is unable to be carried out is brought about in a specific manner, the reproduction of the audio signal ceases and the reproduced audio signal can not be obtained thereafter. This results in the problem that, in the case where sound is reproduced in accordance with the reproduced audio signal, the sound is stopped suddenly and undesirably.